i've solved it, try this way

Press Win+R > type "regedit" > go to "HKEY_LOCAL_MACHINE\SOFTWARE\BlueStacks\Guests\Android\FrameBuffer\0" > Modify the value "Height" check list on decimal then input 720 > Modify the value "Weight" check list on decimal then input 860. ok.

is it working? i hope it's helpful :D